The invention relates to the fields of mechanical engineering and chemistry and refers to a fiber composite material of the type that can be used, e.g., in aircraft and automobiles, and a method for producing the same.
As lightweight materials, fiber composites are exposed to the most varied stresses, but are intended to largely retain their original properties. Among mechanical stresses, dynamic stresses are important, particularly impact stress, as they have a fundamental influence on the durability of the composite material. Thus in multilayer composites the formation of matrix tears and large-area delaminations can occur between layers with different fiber orientation during shock loading perpendicular to the laminate surface. Delaminations in particular should be considered critical damage, since they can easily continue to grow under dynamic material stress, and with compressive load they can lead to buckling of individual layers and thus to the failure of the workpiece.
It is known to reduce the above-mentioned damage during shock loading by giving the mostly very brittle duromer matrix resins, e.g., epoxy resin, a higher fracture toughness by the addition of modifying agents. This is accomplished with various types of modifications.
Flexibilization by the addition of long-chained plasticizers or by the inclusion of long-chained segments in the resin structure (Schichert, P., Rxc3xchlmann, K., Acta Polymerica 38 (1987) 10, p. 562-566).
Dispersion of a small amount of a functionalized elastomer, a polyurethane or a silicone compound in the resin matrix, forming a separate phase (EP 056427, DE 32 24 689 A1, EP 087311, U.S. Pat. No. 5,089,560, U.S. Pat. No. 5,025,045, Hayes, B. S. et al., J. Adv. Mater. 28 (1997) 4, p. 20-25)
Modification of the duromer resins by the addition of thermoplastics, e.g., polyethersulfone or polyetherimide (EP 099338, EP 392348, Pethrick, R. A., et al., Macromolecules 29 (1996) 15, p. 5208-14, Bucknall, C. B. et al., Brit. Polym. J. 15(1993)1, p. 71-75)
The disadvantages of these modifications are increased product costs and a more or less severe reduction in the stiffness and the temperature stability of the composites.
Another possibility for increasing the impact strength of composites is the use of hybrid fiber materials as reinforcing fibers, such as, e.g., hybrid fibers of C- or glass fibers with polyethylene, aramid or polyester fibers (Jang et al. Compos. Sci. Technol. 34(1989)4, p. 305-335).
The disadvantages of this modification are also the expensive production of hybrid filaments and the deterioration of the composite properties, particularly strength and stiffness.
The invention is based on the objective of identifying a fiber composite material and a method for producing the same where substantially reduced delamination is achieved without a significant reduction in other composite properties and without substantial economic disadvantages.
The present invention provides fiber composite material comprising multiple layers, each of the multiple layers containing a duromer matrix and reinforcing fibers, inner layers of the multiple layers containing fibers featuring a higher level of adhesion to duromer matrix, outer layers of the multiple layers containing fibers featuring a lower level or no adhesion to duromer matrix, a number of outer layers being no more than a number of inner layers, and a transition of adhesion between fibers and matrix from the inner to the outer layers being discontinuous or continuous.
The present invention also provides a process for manufacturing a fiber composite material of a multilayered structure, comprising inner layers containing reinforcing fibers featuring a high level of adhesion to duromer matrix and outer layers containing reinforcing fibers featuring a low level or no adhesion to duromer matrix, the process comprising coating the reinforcing fibers, the coating comprising a higher proportion of adhesion promoters for fibers that are to have a higher level of adhesion to the duromer matrix, and the coating comprising a lower or no proportion of at least one of adhesion promoters and additional materials for fibers that are to have a lower level or no adhesion to the duromer matrix, adding the coated fibers separately to a duromer matrix and processing into layers such that layers with fibers with a higher level of adhesion to the duromer matrix are interiorly arranged in the fiber composite material as inner layers, and outer layers are arranged at least one of on and around these inner layers, the outer layers comprising fibers with a lower, low or no adhesion to the duromer matrix, with a discontinuous or continuous structure being employed for the adhesive effect, and forming the fiber composite material wherein a number of the outer layers is no more than a number of the inner layers.
The reinforcing fibers can comprise at least one of silicate fibers and carbon fibers. The silicate fibers can be glass fibers.
The reinforcing fibers can have a coating that generates a good or poor adhesion to the duromer matrix.
The coating can contain an increasing proportion of adhesion promoter with increasing adhesion.
The coating can contain at least one of additional elasticizing components and antiadhesive substances with decreasing adhesion.
The inner layers can be completely enclosed by the outer layers.
The outer layers can be arranged around the inner layers in a symmetrical structure.
The transition of adhesion between fibers and matrix from the inner to the outer layers can be a graduated transition of layers with fibers with good adhesion from inside to outside to layers with fibers with poorer adhesion.
The fibers can be provided with the coating during the fiber formation process.
The solution according to the invention renders possible the disclosure of fiber composite materials that in most cases feature a substantially lower delamination and thus are significantly less susceptible in particular to impact stress.
It is known that composites with good fiber-matrix adhesion are more damage-tolerant and feature lower evidence of delamination particularly under impact stress. However, they are also stiff and brittle.
It is also known that composites with poor adhesion can absorb in particular noticeably more energy as a result of boundary layer failure and fiber-matrix friction. However, they feature only a low level of strength.
According to the invention, both of these phenomena, which are known per se, are achieved together in one composite material, retaining the good qualities, but not the negative qualities.
Due to the structure of the fiber composite material according to the invention, the good and desired adhesion properties of the fibers with the duromer matrix are retained in the interior of the composite. Considerably greater energy absorption and energy distribution occur in the composite under stressxe2x80x94in particular impact stressxe2x80x94in the outer areas with lower or no adhesion of the fibers to the duromer matrix.
Overall, the strength of the composite as a whole is largely retained, without the composite becoming brittle. The tendency towards delamination of the composites according to the invention is noticeably lower.
The adhesive effect of the fibers in the duromer matrix can decrease discontinuously or continuously towards the outside. A continuous decrease is achieved by manufacturing fibers with coatings with a decrease in the amount of adhesion promoters in small steps and/or by adding elasticizing components and/or antiadhesive substances in small steps, using them to make one layer each, and then using a graduated construction in the production of the multilayer composite.